Oxiranes constitute an important class of chemical intermediates useful for the preparation of polyether polyols, glycols, glycol ethers, surfactants, functional fluids, fuel additives and the like. Many different methods for synthesizing oxiranes from the corresponding olefins have been described in the literature. A Japanese patent application assigned to the Tosoh Corporation and published in 1992 (Kokai No. 4-352771) proposed making propylene oxide by reacting propylene, hydrogen and oxygen using a catalyst comprising a Group VIII metal and a crystalline titanosilicate. Improvements to or variations of this basic process were subsequently described in the following published patent applications: WO 97/25143, DE 19600709, WO 96/02323, WO 97/47386, WO 97/31711, JP H8-269030, JP H8-269029, U.S. Pat. Nos. 6,005,123, 6,008,388 and 5,646,314.
As with any chemical process, it would be desirable to attain still further improvements in epoxidation methods of this type. In particular, extending the useful life of the catalyst would significantly enhance the commercial potential of such methods. A problem has been that the noble metal tends to be leached or otherwise lost from the catalyst during use which results in loss of activity and selectivity. Additionally, loss of noble metal imposes an economic penalty which may render the process uneconomic.